Sorry if I wasn't clear. I'm referring to the chapter image. If it happens to be on a transition it will be black. In prior versions of the WB it was not possible for us to precisely set where a chapter point would begin. Though the image would usually show for the chapter in the past, the start point for the chapter was often wrong, especially with chapters toward the end of the video. This was due to a limitation in one of the tools we use in the WB. The tool has now been updated and all chapters are now starting at the correct time code. Because some chapters start during a black transition, the image shown will also be black. Eventually we will tell the WB to take the image from further down the timeline and this will solve this issue.
As for speed, sure the WB will get faster with time. This is a very complex piece of software that is just in the early beta stages. Video codec's and software are changing perhaps faster than anything else technology wise. Just a few years ago the maximum quality image you could play on your iPod was about the same as VHS quality. In just two years we have gone from VHS to HD quality and though this is great, it is also difficult from a software development perspective to keep up with a moving target.
The WB contains code that we have written as well as code that comes from tools and apps the open source community and other companies have written. Every year these other tools and apps that make up the WB improve and become more refined. We've learned to be patient as it is pointless to spend 6 months developing software only to have it made obsolete or pointless by an upgraded app or tool or something Apple does.
You will find that the WB gets faster after the first time you let it load completely as it has to create icons and check a lot of folders on first launch. Eventually this step will no longer be necessary, but for now it is needed.
Something you may not know about the WB is that if you create premixes using video chapters all encoded at the same rate you will be able to create a one hour premix in just a few minutes on a good computer. All of the new Low Impact series are encoded at the exact same specs and thus as long as you create premixes from any of the Low Impact series videos it should only take you a few minutes. Eventually, this will be reduced to just a few seconds, but first the open source tools and apps have to catch up with the technology.